1. Technical Field of the Invention
The present invention relates to the synthesis of hydrocarbon compounds which are fluorinated on a carbon atom of an "alkyl" moiety thereof via exchange between a halogen atom having an atomic number greater than fluorine, employing a fluorine-containing reactant which is at least partially in the form of a complex salt.
This invention more especially relates to a process for the preparation of fluorocompounds that are fluorinated on a carbon atom bearing substituent groups which are electron-withdrawing (or attracting) by means of an inductive effect.
2. Description of the Prior Art
Fluorocompounds are characteristically difficult to prepare. The reactivity of fluorine is such that it is difficult, or even impossible, to directly prepare fluoro derivatives.
One of the most commonly used techniques for preparing fluoro derivatives entails reacting a halogen compound, generally a chloro derivative, and exchanging the halogen with an inorganic fluorocompound, generally an alkali metal fluoride, typically of high atomic weight.
In general, the fluoride employed is potassium fluoride, which provides a satisfactory economic compromise.
Under these conditions, many processes such as, for example, those described in French Certificate of Addition No. 2,353,516 and in the article Chem. Ind., 56 (1978), have been carried out industrially to produce aryl fluorides, onto the aryl moieties of which electron-withdrawing groups are grafted.
Except in the instances where the substrate is particularly suitable for this type of synthesis, this technique presents drawbacks, the principal ones of which are those analyzed below.
The subject reaction requires reactants such as alkali metal fluorides, for example potassium fluoride, which are relatively expensive vis-a-vis the technical specifications they must satisfy in order to be suitable for this type of synthesis; they must be very pure, dry and in a suitable physical state.
In addition, this reaction is not operative for an entire class of compounds, in particular those bearing substituents on the halophoric carbon atom (namely, on the carbon atom bearing the halogen or halogens destined to be exchanged with fluorine).
Reactants such as hydrofluoric acid in liquid form, or diluted with dipolar aprotic solvents, are also used. However, hydrofluoric acid is too powerful or harsh a reactant and often results in unwanted polymerization reactions or in tars.
In this event, and especially in the case where it is desired to obtain derivatives fluorinated on a carbon atom of an alkyl radical (including aralkyl) rendered electron-deficient by the presence of electron-withdrawing groups, this art is faced with an alternative which is not encouraging; either very harsh conditions are selected, and tars are especially obtained, or mild reaction conditions are adopted and, in the best of scenarios, the substrate is unchanged. Lastly, the literature describes exchanges carried out utilizing hydrofluoric acid salt reactants in the presence of heavy elements in oxide or fluoride form. Among the elements thus used, antimony and heavy metals such as silver and quicksilver (mercury) are exemplary.
Another disadvantage is the selectivity of the reaction: when there are more than one halogen to be exchanged on the same carbon atom, it is often difficult to exchange less than all of same.